CYLINDER, VOLUME V
Chapter 3 Scavenging the Two-Stroke Engine design direction acquired by Smyth(3.17) and Kenny(3.31) was applied to the CFD
4.1 The spark-ignition process .1 Initiation of ignition
It is a well-known fact that a match thrown onto some spilled gasoline in the open atmosphere will ignite the gasoline and release a considerable quantity of heat with a significant rise in temperature. The gasoline is observed to burn as a vapor mixed with air above the remaining liquid, but rapidly vaporizing, gasoline. The proce- dure, for those who have witnessed it (although on safety grounds the author is not recommending that the experiment be conducted), commences when the lighted match arrives at the vapor cloud above the liquid, and the ignition takes place with a "whoosh," apparently a major or minor "explosion" depending on the mass fraction of the gasoline which has been allowed to evaporate before the arrival of the match. This tends to leave us with the impression that the gasoline-air mixture within the cylinder of an ic engine will "explode" upon the application of the spark at the sparking plug. That the flammability characteristics of a petrol-air mixture are a decidedly critical phenomena should be obvious to all who have had difficulty in starting either their lawnmower or their automobile!
What then are the requirements of an ignition process? Why does an engine fire up? When does it not fire up? The technical papers on combustion and the engineering textbooks tend to bypass such fundamental concepts, so that this author felt that a paragraph or two would not be amiss, especially as more difficult concepts will only be understood against a background of some fundamental understanding.
Fig. 4.1 depicts a two-stroke engine where a spark has ignited an air-fuel mixture and has produced a flame front burning its way through the mixture. For this to happen, like the match example before it, there had to be a gasoline vapor and air mixture, of the correct mass proportions, within the spark gap when that spark occurred. The energy in the spark provided a local rise in temperature of several thousand degrees Kelvin, which caused any gasoline vapor present to be raised above its Auto-Ignition Temperature. The auto-ignition temperature of any hydro- carbon fuel is that temperature where the fuel now has sufficient internal energy to break its carbon-hydrogen bond structure and be oxidized to carbon dioxide and steam. In the case of gasoline the auto-ignition temperature is about 220QC. The compression process prior to the ignition point helps to vaporize the gasoline, whose maximum boiling point is about 2002C. The mass of gasoline within the spark gap, having commenced to break down in an exothermic reaction, raises the local temperature and pressure. The reaction, if it was stoichiometric, would be as given
Chapter 4 - Combustion in Two-Stroke Engines previously in Eq. 1.5.16. The actual reaction process is much more complex than this, with the gasoline molecule breaking down in stages to methane and aldehydes.
Immediately after the ignition point, the initial flame front close to the spark plug has been established and heats the unburned layers of gasoline vapor-air mixture surrounding it principally by radiation, but also by convection heat transfer and by the mixture motion propelling itself into the flame front. This induces further layers of mixture to reach the auto-ignition temperature and thus the flame front moves through the combustion chamber until it arrives at the physical extremities of the chamber(4.24)(4.25)(4.26). The velocity of this flame front has been re- corded^.3)( 1.20) in two-stroke engines between 20 and 50 m/s. It will be observed that this is hardly an "explosive" process, although it is sufficiently rapid to allow the engine to burn its fuel reasonably efficiently even at the highest engine speeds.
In the example quoted by Kee(1.20), the flame speed, FV, in an engine of similar physical geometry to Fig. 4.1, was measured at 24.5 m/s at 3000 rpm. The longest flame path, DE, from the spark plug to the extremity of the combustion chamber in an engine of 85 mm bore, was about 45 mm. The crankshaft rotation angle, AC, for this flame transmission to occur is given by:
AC=6*RPM*DE/FV (4.1.1) In the example quoted by Kee(1.20), the flame travel time to the chamber
extremity is 33e, as AC is 6*3000*0.045/24.5 from Eq. 4.1.1. That does not mean that the combustion process has been completed in 33s, but it does mean that initiation of combustion has taken place over the entire combustion space for a homogeneous charge. In Sect. 4.2.3, it will be shown that the travel time, in this case cited as 33e, coincides with the maximum rate of heat release from the fuel.
4.1.2 Air-fuel mixture limits for flammability
The flammability of the initial flame kernel has a rather narrow window for success(4.25)(4.26). The upper and lower values of the proportion by volume of gasoline vapor to air for a flame to survive are 0.08 and 0.06, respectively. As one is supplying a "cold" engine with liquid fuel, by whatever device ranging from a carburetor to a fuel injector, the vaporization rate of that gasoline due to the compression process is going to be highly dependent on the temperatures of the cylinder wall, the piston crown and the atmospheric air. Not surprisingly, in cold climatic conditions, it takes several compression processes to raise the local temperature sufficiently to provide the statistical probability of success. Equally, the provision of a high-energy spark to assist that procedure has become more conventional(4.6). At one time, ignition systems had spark characteristics of about 8 kV with a rise time of about 25 (is. Today, with "electronic" or capacitor discharge ignition systems those characteristics are more typically at 20 kV and 4u.s, respectively. The higher voltage output and the faster spark rise time ensure that sparking will take place, even when the electrodes of the spark plug are covered in liquid gasoline.
The Basic Design of Two-Stroke Engines
Fig. 4.1 Initiation of combustion in a spark-ignition two-stroke engine.
Under normal firing conditions, if the fuel vapor-air mixture becomes too lean, e.g., at the 0.06 volume ratio quoted above, then a flame is prevented from growing due to an inadequate initial release of heat. When the spark occurs in a lean mixture, the mass of fuel vapor and air which is ignited in the vicinity of the spark is too small to provide an adequate release of heat to raise the surrounding layer of unburned mixture to the auto-ignition temperature. Consequently, the flame does not develop and combustion does not take place. In this situation, intermittent misfire is the normal experience as unburned mixture forms the bulk of the cylinder contents during the succeeding scavenge process and will supplement the fuel supplied by it.
Under normal firing conditions, if the fuel vapor-air mixture becomes too rich, e.g., at the 0.08 volume ratio quoted above, then the flame is prevented from growing due to insufficient mass of air present at the onset of ignition. As with any flame propagation process, if an inadequate amount of heat is released at the critical inception point, the flame is snuffed out.
4.1.3 Effect of scavenging efficiency on flammability
In Sect. 3.1.5 there is mention of the scavenging efficiency variation with scavenge ratio. As the engine load, or BMEP, is varied by altering the throttle opening thereby producing changes in scavenge ratio, so too does the scavenging efficiency change. It will be observed from Fig. 3.10, even for the best design of two- stroke engines, that the scavenging efficiency varies from 0.3 to 0.95. Interpreting this reasonably accurately as being "charge purity" for the firing engine situation, it is clear that at light loads and low engine rotational speeds there will be a throttle position where the considerable mass of exhaust gas present will not permit ignition of the gasoline vapor-air mixture. When the spark occurs, the mass of vapor and air which is ignited in the vicinity of the spark is too small to provide an adequate release of heat to raise the surrounding layer of unburned mixture to the auto-ignition temperature. Consequently, the flame does not develop and combustion does not take place. The effect is somewhat similar to the lean burning misfire limit discussed above.
During the next scavenging process the scavenging efficiency is raised as the
"exhaust residual" during that process is composed partly of the unburned mixture from the misfire stroke and partly of exhaust gas from the stroke preceding that one.
Should the new SE value prove to be over the threshold condition for flammability, then combustion takes place. This skip firing behavior is called "four-stroking."
Should it take a further scavenge process for ignition to occur, then that would be
"six-stroking," and so on. Of course, the scavenging processes during this intermit- tent firing behavior eject considerable quantities of unburned fuel and air into the exhaust duct, to the detriment of the specific fuel consumption of the engine and its emission of unburned hydrocarbons.
4.1.4 Detonation or abnormal combustion
Detonation occurs in the combustion process when the advancing flame front, which is pressurizing and heating the unburned mixture ahead of it, does so at such a rate that the unburned fuel in that zone achieves its auto-ignition temperature before the arrival of the actual flame front. The result is that unburned mixture combusts "spontaneously" and over the zone where the auto-ignition temperature has been achieved. The apparent flame speed in this zone is many orders of magnitude faster, with the result that the local rise of pressure and temperature is significantly sharp. This produces the characteristic "knocking" or "pinking" sound and the local mechanical devastation which it can produce on piston crown or cylinder head can be considerable. Actually, "knocking" is the correct terminology for what is actually a detonation behavior over a small portion of the combustion charge. A true detonation process would be one occurring over the entire com- pressed charge. However, as detonation in the strictly defined sense does not take place in the spark-ignition ic engine, the words "knocking" and "detonation" are interchanged without loss of meaning in the literature to describe the same effect just discussed.
The effect is related to compression ratio, because the higher the compression
ratio, the smaller the clearance volume, the higher the charge density, and at equal
flame speeds, the higher the heat release rate as the flame travels through the mixture. Consequently, there will be a critical level of compression ratio where any unburned mixture in the extremities of the combustion chamber will attain the auto- ignition temperature. This effect can be alleviated by various methods, such as raising the octane rating of the gasoline used, promotion of charge turbulence, squish effects, improvement of scavenging efficiency, stratifying the combustion process and, naturally, lowering the trapped compression ratio. Some of these techniques will be discussed later.
It is clear that the combustion chamber shape has an influence on these matters.
Plates 4.1 and 4.2 show the combustion chambers typical of loop scavenged and conventionally cross scavenged engines. The loop scavenged engine has a compact chamber with no hot nooks or crannies to trap and heat the unburned charge, nor a hot protuberant piston crown as is the case with the cross scavenged engine. In the cross scavenged engine there is a high potential for fresh charge to be excessively heated in the narrow confines next to the hot deflector, in advance of the oncoming flame front.
Compounding the problem of combustion design to avoid "knocking" in the two- stroke engine is the presence of hot exhaust gas residual within the combustible charge.
Plate 4.1 Compact combustion chamber for a loop scavenged two-stroke engine.
Chapter 4 - Combustion in Two-Stroke Engines
Plate 4.2 The complex combustion chamber of the conventional cross scavenged engine.
4.1.5 Homogeneous and stratified combustion
The conventional spark-ignition two-stroke engine burns a homogeneous charge.
The air-fuel mixture is supplied to the cylinder via the transfer ports with much of the fuel already vaporized during its residence in the "hot" crankcase. The remainder of the liquid fuel vaporizes during the compression process so that by the time ignition takes place, the combustion chamber is filled with a vapor-air-exhaust gas residual mixture which is evenly distributed throughout the combustion space.
This is known as a homogeneous combustion process.
Should the fuel be supplied to the combustion space by some other means, such as direct in-cylinder fuel injection, then, because all of the vaporization process will take place during the compression process, there is a strong possibility that at the onset of ignition there will be zones in the combustion space which are at differing air-fuel ratios. This is known as a stratified combustion process. This stratification may be deliberately induced, for example, to permit the local efficient burning of a small mass of air and fuel of the correct proportions to overcome the problems of
"four-stroking."
It is also possible to utilize charge stratification to help alleviate detonation behavior. If the extremities of the combustion chamber contain air only, or a very
lean mixture, then there exists the possibility of raising engine thermal efficiency with a higher compression ratio, while lowering the potential for detonation to occur.
Further detailed discussion on this topic can be found in Chapter 7, which covers the design of engines for good fuel economy and exhaust emissions.
4.2 Heat released by spark-ignition combustion